Coil coating is a known process for providing paint or film coating to strip metals, such as steel or aluminum, on a continuous basis. The strip metals are typically coated with several layers on both surfaces of the strip metal; and one of the layers is a backer coating layer.
Typically, an epoxy-modified resin has been used in coil backer coating applications. For example, a “type 9” epoxy resin has been widely used to modify a hexa-methoxymethylmelamine (HMMM) cured polyester; and such epoxy-modified polyester resin has been used for coil backer coating applications. Together, the type 9 epoxy resin and the cured polyester resin achieve good performance balance for the coil backer coating in terms of thermal resistance (during curing), chemical resistance, flexibility, and good adhesion to polyurethane (PU) foam. However, the combination of the type 9 epoxy resin and the cured polyester resin has a disadvantage in that both the type 9 epoxy resin and the polyester resin are high molecular weight (e.g., greater than [>]3,800) products which exhibit very high viscosities (e.g., >4,600 mPa-s at 25° C.).
Thus, formulated paints made using the above known epoxy-modified polyester resin have low weight solid content (e.g., less than 60% by weight and about 40% by volume) and high (e.g., >about 420 g/L) volatile organic compounds (VOC). Typically, the weight solid content of the above known epoxy-modified polyester resin is normally less than 50 percent by volume (volume %); and the VOC content of the above known epoxy-modified polyester resin is normally greater than 420 g/L. In view of current environmental protection efforts and regulations, the market demand for high solid (e.g., greater than 50 volume %) coatings with low VOC (e.g., less than 420 g/L is becoming more important in the industry.
Heretofore, some attempts have been made to develop satisfactory high (e.g., >50 volume %) solid coatings with low (e.g., <420 g/L) VOC for coil backer coating applications. For example, CN102993422A discloses an epoxy-modified saturated polyester resin for an undercoat of a coil coating. The epoxy-modified saturated polyester resin includes the following components in parts by weight: 50-80 parts of methylpropanediol, 100-140 parts of epoxy resin 609, 110-150 parts of isophthalic acid, 110-150 parts of hexanedioic acid, 105-145 parts of neopentyl glycol, 0.1-0.6 part of organic tin, 20-40 parts of dimethylbenzene, 380-500 parts of S-150# solvent oil, and 30-60 parts of propylene glycol monomethyl ether.
EP0748830A2 discloses a modified epoxy resin obtainable by reacting an epoxy resin with up to one mol equivalent per epoxy group of an alkyl or alkenyl substituted, hydroxy substituted aromatic acid, more particularly, by reacting a bisphenol A-based epoxy resin with an alkylated salicylic acid.
U.S. Pat. No. 7,812,101 discloses a modified epoxy resin comprising the reaction product of a cycloaliphatic, a polycyclic and/or an aromatic biomass derived compound and an epoxy resin. Aqueous dispersions and coatings comprising the reaction products are also disclosed. The biomass compounds may include a blend of compounds such as abietic acid and cardanol which is the principal component of cashew nut shell liquid (CNSL).
An article by Sultania et al., “Studies on the synthesis and curing of epoxidized novolac vinyl ester resin from renewable resource material”, European Polymer Journal 46 (2010) 2019-2032, discloses synthesizing a cardanol-based epoxidized novolac vinyl ester resin (CNEVER) by reacting a cardanol-based epoxidized novolac (CNE) resin and methacrylic acid (MA) (CNE:MA molar ratio 1:0.9) in the presence of triphenylphosphine as catalyst at 90° C. The CNE resin is prepared by reacting a cardanol-based novolac-type phenolic (CFN) resin and epichlorohydrin in a basic medium at 120° C. The above article teaches using a mono-functional acid (such as MA) during the disclosed synthesis process.